In a digital camera, light is passed through the camera lens to be exposed on an array of pixels having photodiodes such that the captured amount of light in each pixel can be converted into analog electric signals. The analog electrical signal from each pixel is amplified and converted into a digital signal via analog-to-digital conversion. The digital signal is then available for further digital processing to produce the captured image.
An image sensor in a digital camera or other digital imaging system performs the image production process from the pixels capturing light to the digital image output. Therefore, an image sensor is an important component in a digital image system for determining the quality of a final image. CMOS image sensors are widely used in applications requiring high speed performance and low power consumption as CMOS image sensors in general provide higher frame rate operation than the charge coupled devices (CCD). CMOS image sensors can also be formed in a very compact size chip with low power consumption and are used in digital camera implementations in various applications such as mobile phones, personal digital assistants, portable computers, etc.
CMOS image sensors may, however, suffer from high read noise, that is, low signal to noise ratio (SNR), and high fixed pattern noise, that is, low dynamic range (DR). CMOS image sensors provide lower signal to noise ratio (SNR) and smaller dynamic range (DR) than charge coupled devices.
Dynamic range is the ratio between the smallest and largest possible values of a measurable quantity, such as light, expressed as a base-10 logarithmic value (decibel, dB). The dynamic range quantifies the ability to adequately image both high lights and dark shadows in a scene. For example, the human sense of sight has a very high dynamic range, as the human eyes can see objects in bright sunlight and also in a moonless night. The human eye has a dynamic range of around 200 dB.
As the dynamic range of a typical mobile sensor is usually around 60 dB, it appears almost impossible for today's electronic digital imaging equipment to reproduce the full dynamic range experienced by human eyes. Nevertheless, enhancing the dynamic range of the image sensors is one of the research goals of the industry.
Generally, the dynamic range can be extended by adding more hardware on chip to store more light information either in analog domain or digital domain, such as adding more devices in each pixel to increase charge collecting capability, or adding more frame and/or line memory to save images of different exposures, or adding optical masks to vary sensitivities of different pixels. These kinds of hardware solutions have serious limitations in mobile applications because the pixel size is so small that it cannot allow more transistors in the pixel, and the small module size of the mobile devices also limits the size of extra memory on chip, and the small size of the pixels also limits the performance and uniformity of the optical masks.
More specifically, the following techniques including multiple integration, logarithmic sensors, dual sensitivity pixels, and spatial-varying sensitivity pixels have been proposed to improve the dynamic range of the image sensors. However, all of these techniques are considered inadequate.
The multiple integration approach needs extra memory to store full or partial images of different integrations before processing them. The logarithmic sensor approach has high pixel response non-uniformity so that it is bad for color response. The dual sensitivity pixels approach needs extra transistors and/or storage inside the pixel, so the pixel size is hard to shrink.
Spatial-varying sensitivity pixels approach uses different optical masks on different pixels to subsample light information on the same pixel array, and the sensitivity of each pixel cannot be varied at different lighting conditions. Moreover, the pixel response is nonlinear. Accordingly, the pixel response is nonlinear, and for this reason, the pixel response technique requires that the light response of the sensor be calibrated fully and a lookup table be created. The extra filters also add cost.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.